Bacteria in a model biofilm seem to coordinate their activities based on the physical parameters that govern prokaryotic cell-to-cell signaling. The model biofilm is comprised of bacteria that are genetically engineered to transmit and receive quorum-sensing (QS) signals. The model is formed using arrays of time-shared, holographic optical traps in conjunction with microfluidics to precisely position bacteria, and then encapsulated within a hydrogel that mimics the extracellular matrix. Using flourescent protein reporters functionally linked to QS genes, we assay the intercellular signaling. We find that there isn't a single cell density for which QS-regulated genes are induced or repressed. On the contrary, cell-to-cell signaling is largely governed by diffusion, and is acutely sensitive to mass-transfer to the surroundings and the cell location. These observations are consistent with the view that QS-signals act simply as a probe that measures mixing, flow, or diffusion in the microenvironment of the cell.